A conventionally known material for forming microlenses is polyhydroxystyrene. However, there is a room for improvement in the polyhydroxystyrene due to the following disadvantages. Microlenses containing the polyhydroxystyrene as the forming material are readily colored by heat at high temperature and are likely to have impaired transparency, and thus coloring of such microlenses may be recognized when they are used.
In the meantime, a photoresist composition, an anti-reflective coating-forming composition, and a resist protective film-forming material for immersion lithography containing a maleimide-based copolymer, and the like have been proposed (Patent Documents 1 to 4).
In order to develop an insulating film for display devices such as a liquid crystal display (LCD) and an organic electroluminescent display device, a method for forming an insulating film for display devices which is characterized by using a radiation-sensitive resin composition that includes an alkali-soluble copolymer of indene, maleimide, and N-substituted maleimide, a 1,2-naphthoquinone diazide compound, and a cross-linking agent has been proposed (Patent Document 5).
A maleimide copolymer having very excellent compatibility with polyfunctional(meth)acrylate and good alkali solubility has been developed in order to be suitably used for various applications requiring such properties. The maleimide-based copolymer is characterized by including a monomer structural unit derived from unsubstituted maleimides, a monomer structural unit derived from styrenes, and a monomer structure derived from (meth)acrylic acids (Patent Document 6).
However, these related art documents have no object to provide an application for a photosensitive resin composition for microlenses and suggest no specific means for and advantageous effects on the application of a copolymer having an unsubstituted maleimide structural unit and a structural unit including an epoxy group or a blocked isocyanate group to a photosensitive resin composition for microlenses.
A radiation-sensitive resin composition has been reported and is characterized by including [A] a polymer of (a1) an unsaturated carboxylic acid and/or an unsaturated carboxylic acid anhydride, (a2) an epoxy group-containing unsaturated compound, (a3) a maleimide-based monomer, and (a4) other olefinic unsaturated compounds and [B] a 1,2-quinonediazide compound (Patent Document 7). The document reporting the radiation-sensitive resin composition describes that the composition achieves high radiation-sensitivity and can readily form a patterned thin film excellent in solvent resistance, heat resistance, transparency, and heat discoloration resistance, but does not suggest the shape and the heat resistance of a pattern formed of the composition. Moreover, there is no description on an unsubstituted maleimide as the maleimide-based monomer.